Pluto

A color map of the side of Pluto that always faces
Charon. Credit: E.F. Young, R.P. Binzel, and K. Crane

Pluto is a dwarf planet of the subcategory known
as plutoids (see below), smaller than the eight "classical" planets of the Solar System and usually farther
than Neptune is from the Sun. Pluto is only
two-thirds as big as our own Moon and has only
one-fifth of the Moon's mass.

Its size had led to astronomers to question for some time whether Pluto
should be called a planet at all. The debate about Pluto's status intensified
in 2005 when it was shown that Eris (formerly
known as 2003 UB313), an object further out in the Kuiper
Belt, might be larger than Pluto (although a more recent estimate suggests
that Eris is about the same size or slightly smaller than Pluto). On August
24, 2006, at a meeting of the International Astronomical Union (IAU) in
Prague, astronomers voted to downgrade Pluto's status. It is no longer considered
to belong in the same category as Mercury, Venus, Earth, Mars, Jupiter,
Saturn, Uranus, and Neptune. Instead it has been placed in a new category
of "dwarf planets." It has also been given the minor planet designation
of 134340 Pluto. However, this decision to demote Pluto is opposed by several
hundred astronomers. In June 2008, the IAU defined a new subcategory of
dwarf planets to be known as plutoids. A plutoid is any
dwarf planet that orbits beyond Neptune. Only four known objects belong
to this category: Pluto, Eris, Haumea, and Makemake.

Discovery of Pluto

Pluto was discovered by the American astronomer Clyde Tombaugh at the Lowell Observatory in 1930 and
is named after the Roman god of the underworld. The name was first suggested
by an English schoolgirl, Venetia Phair,
and was quickly accepted by the astronomers at Lowell, who also noted that
its first two letters are also the initials of Percival Lowell,
the founder of Lowell Observatory.

To begin with, it was thought that Pluto might be the long-sought-for Planet
X, a world beyond the orbit of Neptune that was believed to be disturbing
the movements of Neptune and Uranus. But
astronomers quickly realized that the mass of Pluto was much too low to
have any noticeable effect on the orbits of its inner neighbors. Tombaugh
had been searching in a part of the sky where his calculations indicated
that Planet X might lie. However, it's now known that there are no unexplained
perturbations in the orbits of Neptune and Uranus. Tombaugh's discovery
of Pluto, by use of a blink comparator,
turned out to be just a lucky accident.

Pluto's orbit

Pluto has a far more eccentric – stretched-out – orbit than
any of the eight classical planets of the Solar System. Most of the time,
Pluto is much farther from the Sun than is Neptune. However, for about 10
years on either side of perihelion its elongated
orbit carries it closer to the Sun than Neptune.
The last time this happened was at the end of the 20th century, from January
21, 1979, to February 11, 1999 (perihelion was on September 5, 1989).

Pluto's orbit is also unusual in being highly inclined and in having a 3:2 resonance with that of Neptune,
so that Pluto completes three orbits for every two of Neptune. There is
no danger, though, that these two worlds will ever collide. As Pluto approaches
perihelion, it also reaches its maximum distance from the ecliptic due to its 17° orbital tilt, so that it is always far above or below
the plane of Neptune's orbit. In fact, Pluto and Neptune never come closer
than 18 astronomical units (AU) apart.

Rotation

Unlike most of the eight classical planets, but similar to Uranus,
Pluto spins on its side – that is, with its poles almost in its orbital
plane. This extreme axial tilt, together with the high orbital tilt, makes
for pronounced seasonal climatic changes – though, of course, it is
always very cold!

Atmosphere

Pluto has very thin atmosphere with a surface pressure 100,000 times less
than that on Earth. It is composed of 98% nitrogen and small amounts of
methane and carbon monoxide – much like the atmosphere of Neptune's
moon Triton.

Pluto's atmosphere gradually freezes and collects on the surface as the
planet moves away from the Sun. Yet, interestingly, observations have shown
that between 1989, when Pluto was at perihelion, and 2002, the atmospheric
pressure increased threefold. The explanation probably has to do with the
fact that materials take time to warm up and cool off, which is why the
hottest part of the day on Earth, for example, is usually around 2 or 3
p.m. rather than local noon, when sunlight is the most intense. The fact
that Pluto's atmosphere is still building up rather than freezing out, as
many scientists expected, is good news from the standpoint of what the New
Horizons probe, launched toward Pluto in 2006, might learn of this enigmatic
world.

Surface and interior

The structure of Pluto is not very well understood at present. The accompanying
diagram shows one possibility – a very small metal core surrounded
by a thick mantle of water-ice.

Spectroscopic observation from Earth in the 1970s has revealed that the
planet surface is covered with methane ice. Surface temperature is about
-230°C, and the frozen methane exhibits a bright coloration. However,
with the exception of the polar caps, the frozen methane surface is seen
to change to a dark red on the basis of observation of eclipses by its moon
Charon.

The moons of Pluto

Pluto has five known satellites. The largest of these, Charon,
is so big in comparison with Pluto – more than half its diameter –
that the Pluto-Charon system is often referred to as a double planet. The
average separation distance is 19,640 kilometers (roughly eight Pluto diameters).

Uniquely, among planets in the solar system, Pluto has a rotation period
exactly equal to that of one of its moons (Charon). Although it is common
for a satellite to travel in a synchronous orbit with its planet, Pluto
is the only planet to rotate synchronously with the orbit of its satellite.
Thus tidally locked, Pluto and Charon keep the same faces toward each other
as they travel through space. From 1985 to 1990, Earth was aligned with
the orbit of Charon around Pluto enabling daily observations of the mutual
eclipses of the pair. The data collected showed that Pluto has a highly
reflective south polar cap, a dimmer north polar cap, and both bright and
dark features in the equatorial region. Its geometric albedo ranges from 0.49 to 0.66, compared with Charon's (darker) 0.36 to 0.39.
This difference, together with Pluto's higher density (between 1.8 and 2.1
g/cm³ compared with Charon's 1.2 to 1.3 g/cm³), suggests that
the two bodies formed separately rather than as the result of a single collision,
though this is by no means certain. Pluto's higher density indicates it
is made of 50% to 75% rock mixed with ices, whereas Charon seems to be mostly
ice with very little rock.

Two small outer satellites were discovered in 2005, a third in 2011, and
a fourth in 2012. See Pluto, moons.

Pluto's origin

Several odd facts about Pluto, especially its highly inclined and eccentric
orbit and its small size, led to early speculation that it might be a moon
of Neptune that escaped. The modern view, however, is that Pluto is a displaced Kuiper Belt object.

Spacecraft

Pluto is the only planet unvisited by spacecraft. However, this will change
in 2015 when the New Horizons probe
flies through the Pluto-Charon system.